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Nanocrystalline cellulose (NCC): A renewable nano-material for polyvinyl acetate (PVA) adhesive
European Polymer Journal 48 (2012) 1829–1837
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Macromolecular Nanotechnology
Nanocrystalline cellulose (NCC): A renewable nano-material for polyvinyl acetate (PVA) adhesive
a Département des sciences du bois et de la forêt, Faculté de foresterie, de géographie et de géomatique, Université Laval, 2425, rue de la Terrasse, Québec, QC, Canada G1V 0A6 b Nanotechnologies for Wood Products, FPInnovations, 319 rue Franquet, Québec, QC, Canada G1P 4R4 c CNR IVALSA, Consiglio Nazionale delle Ricerche – Istituto per la Valorizzazione del Legno e delle Specie Arboree I – via Biasi 75 – 38010 S. Michele all’Adige (TN), Italy d Center for Renewable Carbon, University of Tennessee, 2506 Jacob Drive, Knoxville, TN 37996-4570, USA
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 3 August 2012 Accepted 13 August 2012 Available online 19 August 2012 Keywords: Durable wood adhesives Polyvinyl acetate (PVA) Nanocrystalline cellulose (NCC) Wood joints Nanoindentation
a b s t r a c t In this study nanocrystalline cellulose (NCC) was used to improve the performance of polyvinyl acetate (PVA) as a wood adhesive. NCC was added to PVA at different loadings (1%, 2% and 3%) and the blends were used as binder for wood. Block shear tests were done to evaluate bonding strength of PVA at different conditions; dry and wet conditions, at the elevated temperature (100 °C). The mechanical properties of PVA film and its composites with NCC were measured by nanoindentation technique. Thermal stability and structure of nanocomposites were studied by thermogravimetric analysis and atomic force microscopy (AFM). The block shear tests demonstrate that NCC can improve bonding strength of PVA in all conditions. Hardness, modulus of elasticity (MOE) and creep of PVA film were also changed positively by the addition of NCC. Thermal stability of PVA was significantly improved as NCC was added to PVA. Structural studies revealed that variations in shear strength and other properties can be related to the quality of NCC dispersion in the PVA matrix. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently there has been a strong tendency to use so called ‘‘green materials’’ for the industry. The tendency has forced the industry to look for renewable materials with the least impact on the environment. One of the industries that can benefit from this trend and subsequently experience significant and constant growth is wood industry. Wood can provide the market with the prospects which cannot be offered by the other materials. Wood has a great potential to become the dominating material for the construction. As a material for the construction, wood has unique properties; such as high strength, flexibility, fire ⇑ Corresponding author. E-mail address: [email protected] (B. Riedl). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.08.008
resistance, durability, isolation and also not having carbon finger print. Introduction and development of engineered wood products (EWPs), which can be produced in different shapes and sizes, has opened new markets for wood industry and expand the current markets. Adhesives play an important role on the performance of EWPs. Production of EWPs with high performance needs durable adhesives. Commercial durable wood adhesives contain formaldehyde, causing concerns on the health during the production and the service of EWPs. Wood industry is under increasing pressure to eliminate formaldehyde from its products. Hence, wood industry is looking for alternatives. Polyvinyl acetate (PVA) is a good alternative to replace some wood adhesives containing formaldehyde. PVA is a linear and thermoplastic polymer. It is watersoluble, biodegradable with the excellent chemical
MACROMOLECULAR NANOTECHNOLOGY
Alireza Kaboorani a, Bernard Riedl a,⇑, Pierre Blanchet a,b, Marco Fellin c, Omid Hosseinaei d, Sequin Wang d
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resistance and has no toxic action on the human body. As a wood adhesive, utilization of PVA is very simple and its curing does not need high temperatures. The main drawback of PVA is its weak performance towards humid conditions and elevated temperatures. These drawbacks limit the usage of PVA in the applications where high performance is required in humid conditions and at elevated temperatures. So far two approaches have been used to increase the performance of PVA: (1) Copolymerizing vinyl acetate with more hydrophobic monomers or functional monomers [1–3], and (2) blending PVA with other adhesives or hardeners [4–13]. These strategies can increase some properties of PVA at the expense of reducing some other properties. Moreover, some additives are so acidic that they can damage wood subtracts, finally affecting the overall performance of wood joints. For instance, copolymers of vinyl acetate and butyl acrylate or ethylene can increase water resistance and toughness of the adhesive, but will reduce its tensile modulus and stiffness, especially at elevated temperatures [12]. The introduction of nanotechnology has opened new opportunities for the industries to develop a new generation of composites with high performance. Past research has showed that nano-clay and nano-aluminum oxide particles can be used to boost the performance of wood adhesives [14,15]. There has been always concerns on the health risks posed by nano-clay and nano oxide metal particles, however. Also the renewability of these kinds of nano-particles has been always under question. Such concerns gave a momentum to introduction of renewable and more eco-friendly nano-materials. Cellulose is the most abundant organic polymer in the biosphere and the main constituent of wood. Cellulose has a nano-structure which can be employed in nanotechnology. Cellulose fibrils consist of different hierarchical microstructures commonly known as nano-sized microfibrils. These nano-sized fibrils are again made-up of a combined crystalline and amorphous parts and this crystalline region is named in the literature nanocellulose or nanowhisker [16]. Nanocrystalline cellulose (NCC) is obtained by the acid hydrolysis of cellulose under conditions where the amorphous regions are selectively hydrolyzed. For woodbased NCCs, the remaining crystalline regions are 3– 10 nm in diameter and 100–300 nm long and retain the natural cellulose I crystalline structure [17,18].Among their interesting characteristics, NCCs are abundant and renewable. They exhibit also a low density as compared to mineral fillers (around 1.5 g cm3) and a high form factor of about 70 together with a high specific area of 150 m2 g1. They have been shown to lead to remarkable reinforcing properties in such different matrixes as styrene–acrylate latex [19], starch [20], polyhydroxybutyrate octanoate [21] or poly(ethylene oxide) [22]. In most cases, the reinforcing effects came from the percolating network of NCC together with good interfacial compatibility between the matrix and the fillers. The hypothesis of this study is that using NCCs as reinforcing nano-materials improves the performance of PVA as a wood adhesive. Such improvement can expand the applications of PVA, especially in the wood construction domain in which durable materials with high performance
are required. The main objective of this research is to develop durable eco-friendly wood adhesives with an application in construction field. The effects of adding NCCs on the mechanical properties, morphology and thermal stability of PVA will be discussed in this paper. 2. Materials and methods 2.1. Materials A commercial grade of polyvinyl acetate (PVA) was received in liquid form. Nanocrystalline cellulose (NCC), provided kindly by Forest Products Laboratory in Madison, WI. USA was used as reinforcing material, (in 5.5% suspension). Black spruce (Picea mariana Mill.), which were obtained from trees grown in province of Québec, Canada was used as wood substrates. 2.2. Preparation of nanocellulose/PVA composites A certain amount of the suspension of NCC was added to PVA, depending on the percentage of nanocellulose in PVA. The blends of PVA with nano-cellulose were mixed for 30 min. For TGA, nanoindentation and AFM, samples of nanocomposites were prepared by casting the NCC– PVA composites on Teflon sheets. Prior to further analyses, the sheets of nanocomposites were allowed to dry at room temperature for at least two weeks. 2.3. Fabrication and tests of wood joints Blends of PVA and NCC were used to bond wood joints. Black spruce was used as substrates. Prior to gluing, the moisture content of wood was fixed at 12% by conditioning wood to 20 °C and 60% relative humidity for four months. After applying glue on the surface of wood, the samples were pressed in an MTS hydraulic test machine with 50 kN capacity at 2.46 kg/cm2 pressure for three hours. Before testing, glued samples were conditioned to 20 °C and 60% relative humidity for two weeks. 20 samples were tested for each set of formulation. Shear strength of wood joints was measured in dry and wet states, and at an elevated temperature. An MTS hydraulic test machine with 50 kN capacity was used for load application and the data was acquired by a computer. Wood failure and maximum load were recorded for each test. The block shear tests were carried out according to ASTM D905-98. The sizes of samples for ‘‘wet state’’ tests were the same as those for dry state tests. For ‘‘wet state’’ tests, the samples were taken directly out of the water after being immersed in water for 24 h. Before the tests, excess water was wiped off from the samples. During the water immersion period, temperature of water was maintained at 23 ± 1 °C. Block shear tests at the elevated temperature were carried out according to ASTM 7247-07. Samples made of black spruce were heated in an oven, having a temperature controller with an integral and derivative (PID) control algorithm, until the temperature of middle of samples reached 100 °C. On average, it took 30 min to reach
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2.4. Characterization of the nanocellulose/PVA composites
Er ¼
2.4.1. Nanoindentation Oliver and Pharr in 1992 introduced a technique, nanoindentation, to characterize the localized material properties [23]. Conventional measurements of mechanical properties only give a value for each test. In return, nanoindentation technique provides an opportunity to measure the mechanical properties of composites materials in desired areas. In this technique, the areas under test can be selected, giving different values depending on the place of tested areas. A typical nanoindentation profile for determination of localized mechanical properties in polymer composites is shown in Fig. 1. In the technique, a permanent plastic deformation is created in the surface of a material being tested. Hardness, elastic modulus (MOE) and creep of the material can be calculated by using the load-displacement data from the nanoindentation tests. For an indentation depth (h), the hardness (H) of a sample can be calculated from the following equation:
H¼
Pmax A
ð1Þ
where Pmax is the load measured at a maximum depth of penetration (h) in an indentation cycle, while A is to the projected area of contact between the indenter and sample at Pmax. In theory, the elastic modulus should be size independent [24], but analyzing polymers by NI causes a number of complications summarized under the term indentation size effect, which means that the elastic modulus tends to increase with decreasing penetration depth of the indenter pyramid. Also viscoelastic creep during unloading may affect the slope of the unloading curve, and thus the
Fig. 1. The loading profile of nanoindentation.
calculated elastic modulus. Comparing elastic moduli (E) of polymers from bulk measurements and indentation tests, results often show that Ebulk < Esurface [25]. From the initial slope of the unloading curve the unloading stiffness (S) is determined and the reduced elastic modulus Er is calculated according to Eq. (2).
1 pffiffiffiffi S p pffiffiffi 2 A
ð2Þ
Er is termed the reduced elastic modulus because it takes into account the compliance of the indenter tip according to Eq. (2).
1 m2i 1 1 m2m ¼ þ Er Em Ei material indenter
ð3Þ
The elastic modulus Em of the specimen was calculated according to Eq. (3) using Poisson’s ratios m determined in a previous study [26]. Ei and mi stand for the elastic modulus and the Poisson’s ratio of the indenter. Since the compliance of diamond is very small compared with that of the tested polymers, the term representing the indenter properties (subscript indenter) in Eq. (3) was disregarded.
C IT ¼
h2 h1 100 h1
ð4Þ
Indentation creep CIT (Eq. (4)) [27] was defined as the relative change of the indentation depth while the applied load remains constant (Fig. 1). To prepare the samples for the nanoindentation test, pure PVA and its composites with NCC were embedded in Spur epoxy resin. Spur epoxy resin is used as an embedding medium for electron microscopy of biological samples. The curing was performed in a vacuum oven at 70 °C for 7 h. An ultramicrotome (Leica, Vienna, Austria) was used to make a very smooth surface, by avoiding creating any default, on the surface of sample. A five-minute epoxy adhesive was used to glue the resin-embedded samples to an acrylic block. The acrylic block was then mounted onto an ultra-microtome. First a glass knife was used to level surface of the samples and then further on, creating a smooth surface was completed by a diamond knife. All nanoindentation experiments were performed on a Triboindenter (Hysitron, Minneapolis, MN) equipped with a three-sided pyramid diamond Berkovich tip. Before nanoindentation tests, samples were conditioned at 21 °C and 60% relative humidity for at least 24 h. Indentation was performed in a load-controlled mode using three segments. The loading time, holding time, unloading time and maximum indentation force (Pmax) are 5 s, 60 s, and 5 s and 150 lN for all testing samples, respectively. The separation distance between each nanoindentation point was 0.05 mm. 2.4.2. Thermal stability The thermal stability of pure NCC, pure PVA and their nanocomposites was studied using a thermogravimetric analyzer, Mettler Toledo TGA/SDTA 851e, on 13–15 mg of samples over a temperature range of 25–700 °C at a heating rate of 10 °C/min under a nitrogen flow rate of 50 mL/min. In order to avoid unwanted oxidation, thermal
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100 °C in the middle of samples. After reaching 100 °C in the middle of samples, the samples were kept at 100 °C for 15 min more, followed by the immediate block shear test. The shear strength of sample was measured by an MTS hydraulic test machine.
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stability measurements were conducted with the samples placed in high quality nitrogen (99.5% nitrogen and 0.5% oxygen content) atmosphere.
Ra ¼
n 1X jZi Zav ej n i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðZi Zav eÞ2 Rq ¼ RMS ¼ n
ð5Þ
ð6Þ
where Ra is the mean roughness, the arithmetic average of the absolute values of the surface height deviations, and Rq is the root mean square of the height and in both equations Zi is the current Z value, Zave is the average of the Z values within the given area and n is the number of points within the given area.
3.1. Bonding strength Fig. 2 shows shear stress at failure of wood joints bonded by pure PVA and its composites with NCC. Adding NCC to PVA did increase the stress but not in a significant way. Although not being significant in terms of the stress at failure, adding NCC to PVA significantly increased the wood failure, proving its positive effects on bonding
Shear stress [MPa]
80
6
60
4
40
2
20
Pure PVA
NCC 1%
NCC 2%
NCC 3%
0
Blend name Fig. 2. Shear strength and wood failure of joints bonded by PVA and its composites with NCC in dry state.
strength of PVA glue line. Wood failure value increased from 59% for pure PVA to 84% for adhesive with 1% NCC and finally to 97% for adhesive with 3% NCC. Such an increase shows that NCC made the glue line of PVA stronger than wood, causing failure in the wood rather than in the glue line. The loading level of NCC affected the performance of PVA. Wood failure percentage increased as more NCC was added to PVA. Sriupayo et al. [28], Kvien and Oksman [29], Zimmermann et al. [30], Favier et al. [31], Gong et al. [32] , Dalmas et al. [33,34], Mathew et al. [35] and Lee et al. [36] reported that nanocellulose positively affected the strength of the polymers. Favier et al. [31] stated that the intermolecular forces between nanocellulose and the PVA matrix may contribute to enhancement of strength of the PVA composite films. The intermolecular forces keep the inherent strength of the fibrils intact and results in enhancement of the mechanical strength of the films. Some reports have attributed the strong reinforcing effect of the NCC to the formation of a networked structure above percolation threshold resulting from hydrogen bonding [37,38].
8
100
7
Stress
90
Wood failure
80
6
70
5
60
4
50
3
40 30
2
20
1 0
Wood failure [%]
3. Results
8
0
2.5. Statistical analyses A one way analysis of variance model was used to study the effects of NCC loading on shear strength of wood joints. The GLM procedure of the SAS program was used and pair wise comparisons were then made using protected Fisher LSD (Least Significant Difference).
100
10
Shear stress [MPa]
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2.4.4. Atomic force microscopy (AFM) AFM observations were carried out using a NanoScope V, an atomic force microscope (Veeco Instruments Inc.), fitted with a Hybrid XYZ scanner. Atomic force microscopy measurements were done under ambient air conditions in tapping mode. The sensitivity of the tip deviation and the scanner resolution was 0.3 nm. The resolution was set to 250 lines by 256 pixels for all observations. Surface roughness was calculated in 10 lm 10 lm scan areas, using the classical mean surface roughness parameters Ra and Rq (RMS). The parameters were calculated by the Research Nanoscope 7.2 software:
120
Wood failure
Wood failure [%]
2.4.3. Transmission electron microscopy High-resolution transmission electron microscopy (TEM) was carried out with a JEOL 100 CX II microscope at 100 kV. The samples for TEM observations were prepared from a solution with 0.5% NCC. The samples were prepared by casting a drop of diluted aqueous dispersion onto a 200 mesh holey copper grid covered with a carbon film. To improve the contrast, the cellulose nanofibers were stained with a 1 wt.% solution of uranyl acetate for 30 s. The sample was then dipped into a droplet of distilled water for 2 s to remove the excess uranyl acetate. It was then allowed to dry at ambient temperature overnight.
Stress
10 Pure PVA
NCC 1%
NCC 2%
NCC 3%
0
Blend name Fig. 3. Shear strength and wood failure of joints bonded by PVA and its composites with NCC in wet state.
A. Kaboorani et al. / European Polymer Journal 48 (2012) 1829–1837
100
4 Wood failure
90
Studying the response of composites materials to a mechanical load in nano-scale helps to develop a better understanding of impact of nano-materials on mechanical properties of nanocomposites. Nanoindentation technique can provide us with an opportunity to understand the response of composites to a load in a nano-scale. Values of hardness and their variations in pure PVA and its composites with NCC are shown in Fig. 5. NCC addition to adhesive increased average values of hardness significantly. Adding only 1% NCC to PVA increased average value of hardness by 70%. At 2% NCC loading, the extent of improvement decreased and reached only 55%. An increase in NCC loading form 2% to 3% resulted in a huge increase in hardness (155%, more than double). Fluctuations in hardness within the composites were also affected by NCC. Variations in hardness were not very high in pure PVA. As NCC was inserted into PVA, the variations in hardness became high. The extent of variations in hardness was related to amount of the NCC in PVA. The higher the ratio of NCC to PVA, the greater the variation in hardness. Average values of MOE and their variations for each formulation measured by nanoindentation technique are shown in Fig. 6. Average values of MOE were affected significantly by addition of NCC. Adding 1% NCC to PVA increased average value of MOE by 48% but adding more NCC (2%) to PVA did not change MOE considerably. An increase in NCC loading from 2% to 3% resulted in an important increase in MOE. Film with 3% NCC showed the highest value of MOE. The variations of MOE in different parts of composites were influenced by adding NCC as well. The values of MOE in pure PVA were pretty constant. As NCC was added to the PVA matrix, the variations in MOE became more important. The highest variations in MOE were found in composites with 3% NCC. One of drawbacks of PVA is its high creep. Nanoindentation technique provides an opportunity to measure creep in different parts of film and analyze the effects of different fillers on performance of the polymer. Creep measurements conducted on the film of PVA and its composites demonstrated that NCC has a significant effect on average values of creep (Fig. 7). Adding NCC to PVA reduced the creep. As the percentage of NCC in PVA increased, the effect of NCC in reducing creep became more prominent. Adding
80
3
70 60 50
2
40 30
1
Wood failure [%]
Shear stress [MPa]
Stress
3.2. Nanoindentation
20 10
0
0
Pure PVA
0
0
NCC 1%
NCC 2%
0
NCC 3%
0
blend name Fig. 4. Shear strength and wood failure of joints bonded by PVA and its composites with NCC at the elevated temperature.
Fig. 5. Average values of harness for different PVA–NCC composites.
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The values of shear stress at failure of wood joints after spending 24 h in water are shown in Fig. 3. Nanocellulose affected significantly shear strength of wood joints after spending 24 h in water. Adding only 1% NCC to PVA increased the strength by 63%. Further increase in NCC (from 1% to 2%) increased shear strength of wood joints but on a much smaller scale. An increase in NCC loading from 2% to 3% did not change the strength much. The improvement on water resistance of PVA was so high that some failure occurred in glue line instead of wood. As more NCC was inserted in PVA, the percentage of wood failure increased, reaching its maximum value at 3% NCC level (21%). Sriupayo et al. [28], Choi et al. [39], Wang et al. [40], Lu et al. [41] and Dufresne et al. [42] reported a reduction in water uptake and diffusion coefficient of the polymers as nanocellulose added to the polymers. The high crystallinity of cellulose might be responsible for the decreased water uptake at equilibrium and the decreased diffusion coefficient of the materials. The presence of NCCs in the polymer can also create tortuosity in the nanocomposites films, leading to slower diffusion processes and, hence, to a lower permeability [43]. Fig. 4 shows the values of shear strength at 100 °C. NCC affected PVA performance at the high temperature significantly. Adding NCC to PVA doubled the shear strength of PVA. There was no significant difference between different loading levels of NCC. Adding only 1% NCC to PVA resulted in a 100% increase in shear strength and further addition did not change the strength much. Angles et al. [20] and Favier et al. [31] reported that incorporation of NCC in composites resulted in a significant increase in mechanical properties, especially at temperatures greater than the glass transition temperature. They attributed the reinforcing effect to the formation of a rigid NCCs network governed by the percolation threshold and strong hydrogen bonding between NCCs. It was demonstrated that the reinforcing effect of NCC lies in the formation of a rigid percolating filler network, due to hydrogen bond interactions between NCCs [44–46].
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Fig. 6. Average values of MOE for different PVA–NCC composites.
Fig. 7. Average values of creep PVA and its composites with NCC.
3% NCC led to 55% reduction in creep. Variations of creep were also affected by NCC. Pure PVA had a high variation in creep throughout the matrix. In the formulations with NCC, the variations in creep were less. The smallest variations in creep were observed in the formulation with 3% NCC. The results of measuring hardness, MOE and creep by nanoindentation technique demonstrate the great influence of NCC on the mechanical properties of PVA matrix. NCCs create an inhomogeneous matrix with different localized mechanical properties. Such a matrix shows improved mechanical properties, capable of bearing higher mechanical load as shown by bonding strength tests in this study. 3.3. Thermal stability Mass loss of PVA and its composites (measured in the range of temperature) was used as a tool to evaluate the effects of NCC on thermal stability of PVA. Thermal stability of PVA was affected by NCC (Fig. 8). The direction of changes in the thermal stability depended on the temperature. At temperatures less than 340 °C, PVA with NCC had better thermal stability than pure PVA. In temperatures between 340 and 420 °C, PVA and its composites had almost the same thermal stability. At temperatures higher than 420 °C, NCC negatively affected the thermal stability of PVA. No significant difference was detected between the nanocomposites having different amounts of NCC at temperatures less than 420 °C. Thermal stability of pure NCC
Fig. 8. Mass loss (a) and its rate (b) for PVA and its composites.
was similar to PVA composites in temperatures less than 235 °C. Between 235 and 340 °C, thermal stability of pure NCC is less than its composites with PVA. Thermal stability of pure NCC was better than its composites at temperatures higher than 340 °C. To have a detailed view over mass loss pattern occurred during thermal stability tests, the dM/dT curves obtained by derivation of mass loss/temperature data are given in Fig. 8b. The dM/dT curves show the speed of mass loss during thermal stability tests. Pure PVA has three peaks of weight loss at 258, 331 and 436 °C. As a result of adding NCC to PVA, one of the three peaks located at 258 °C disappeared. The second and biggest peak, located at 331 °C, remained almost untouched but the third peaks located at 436 °C occurred at little bit higher temperatures. It has
Table 1 Values of ash contents and DTGmax for pure PVA, pure NCC and their composites. Samples
DTGmax (°C)
Ash content (%)
Pure PVA Pure NCC PVA & 1% NCC PVA & 2% NCC PVA & 3% NCC
331.1 305.6 335.5 337.0 337.8
13.2 15.8 10.3 8.9 9.1
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been reported that the formation of a rigid network, resulting from strong interactions between NCCs and the polymer (governed by a percolation mechanism) [19,31], induced a thermal stabilization on the composite up to 227 °C, the temperature at which cellulose starts to
decompose. For rod-like particles with an aspect ratio of 67, the percolation threshold is close to 1 vol.% [47]. The formation of this cellulose network was supposed to result from strong interactions between NCCs, such as hydrogen bonds [48]. This mechanical percolation effect explains both the reinforcing effect and the thermal stabilization of the composite modulus for evaporated films. Values of DTGmax degradation temperature and ash contents for PVA and its composites are given in Table 1. DTGmax degradation temperature is a temperature in which the highest weight loss takes place (peak point in dM/dT curve). Pure PVA has higher DTGmax than pure NCC. Composites of NCC and PVA have higher DTGmax than pure PVA and pure NCC. It means that NCC can change the response of PVA towards temperatures. Ash content of PVA was also affected by adding NCC. Composites with NCC have lower ash contents than pure PVA. NCC proportion in PVA inversely affects the ash content of PVA. 3.4. Transmission electron microscopy Fig. 9 shows a TEM image of dilute suspension of NCCs. The NCCs have a rod-like structure. Acrylic latexes have been employed for a variety of applications including
Fig. 10. AFM amplitude mode analysis of PVA films with (a) 0%, (b) 1%, (c) 2 and (d) 3% NCC.
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Fig. 9. TEM image of NCCs.
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painting and adhesion. The suspension exhibited a colloidal behavior. This is due to the presence of the positive charges (NH3+) on the NCC surface, which resulted from the protonation of the amino groups of chitin in acidic conditions [49]. The suspension contains fragments consisting of both individual microcrystals and aggregated microcrystals. The rod-like NCCs were shown to have length values of 150–250 nm and width on the order of 4–10 nm, leading to an aspect ratio of 38–62.
NCC led to agglomeration of NCC and difficulty in dispersion of NCC in PVA at high loadings (2% and 3%). The agglomeration limits the extent of improvement that be achieved by NCC. Modifying the surface of NCC a little bit and imposing a slightly hydrophobic character on the surface can help to prevent agglomeration of NCCs and to have a better NCC dispersion in the matrix at a high loading. The modification of surface should be done in controlled conditions without making NCC too hydrophobic or degrading its reinforcing effects.
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3.5. Atomic force microscopy (AFM) To further explore the fundamental interactions between NCC and PVA, composite of PVA containing different loading of NCC (0%, 1%, 2% and 3%) were analyzed by atomic force microscopy (AFM). The AFM images for the composites are presented in Fig. 10. The images clearly demonstrate a re-organization of the surface of the composite films. Pure PVA film has a smooth surface as it has low Ra and Rq (Table 2). Adding 1% NCC to the matrix did not change Ra and Rq considerably, showing good dispersion of NCC at 1% loading. Adding 2% and 3% NCC to the matrix re-organized the surface as big jumps in Ra and Rq were detected. Such high Ra and Rq are evidence for poor quality of NCC dispersion in the matrix, and presence of aggregates. Seemingly high interaction between PVA and NCC (in the form of hydrogen bonding) prevented NCCs to be dispersed uniformly in the matrix. Comparing the results of morphological studies and mechanical and thermal properties displays that thermal stability and bonding strength (in wet conditions and at the elevated temperature) have more sensitivity to the quality of NCC dispersion in the PVA matrix. At 1% loading, NCC improved the thermal stability and bonding strength of PVA significantly. But adding more NCC (2% and 3%) did not increase the properties beyond the extent obtained by 1% addition. This trend is related to the quality of NCC dispersion. At 1% NCC loading, significant improvement on bonding strength and thermal stability was achieved as quality of NCC dispersion was good. At 2% and 3% NCC loadings, dispersion of NCC in PVA encountered difficulty, as shown by the AFM results, and a poor dispersion of NCC in the matrix was obtained leading to a leveling-off of the improvement. In the context of the others studies conducted on the reinforcement of PVA with inorganic nano-materials [13,14], the extent of improvement achievable by NCC is higher than that of inorganic nanomaterials, namely nano-clay and aluminum oxide. The improvement is especially much higher for thermal stability and bonding strength at wet conditions. In this study unmodified and hydrophilic NCC was used to reinforce PVA. Using such unmodified and hydrophilic Table 2 Values of surface roughness for pure PVA and its composites with NCC. Samples
Rq (nm)
Ra (nm)
Pure PVA PVA & 1% NCC PVA & 2% NCC PVA & 3% NCC
39.9 27.6 140 155
31.7 21.6 113 120
4. Conclusions NCC was found to be an effective nano-reinforcing material for PVA. Bonding strength of wood joints was improved significantly as NCC was inserted to PVA. The improvement was measurable in terms of wood failure percentage in dry condition and in terms of shear strength in wet condition and at the elevated temperature. NCC increased hardness and MOE of PVA film significantly. The creep experienced a big reduction as NCC was added to the PVA matrix (especially at high loadings of NCC). Thermal stability of PVA gained big enhancement by the addition. The three levels of NCC loading affected thermal stability in the same extent. Structural study of NCC–PVA composites revealed that NCC changed the structure of PVA. Fluctuations in properties of PVA as a wood adhesive and a polymer at different loadings of NCC could be explained by the quality of NCC dispersion in the matrix. The thermal stability and bonding strength of wood joints showed more sensibility to the quality of NCC dispersion in the matrix.
Acknowledgements The authors acknowledge financial support from Natural Sciences and Engineering Research Council of Canada. The authors would like to express their thanks to Forest Products Laboratory – USDA Forest Service for so kindly providing nanocrystalline cellulose (NCC) for this research. Thanks are also extended to FPInnovations for their assistance.
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[30] Zimmermann T, Pohler E, Geiger T. Cellulose fibrils for polymer reinforcement. Adv Eng Mater 2004;6(9):754–61. [31] Favier V, Canova GR, Cavaille JY, Chanzy H, Dufresne A, Gauthier C. nanocomposite materials from latex and cellulose whiskers. Polym Adv Technol 1995;6(5):351–5. [32] Gong G, Mathew AP, Oksman K. Toughening effect of cellulose nanowhiskers (CNWs) on polyvinyl acetate (PVAc): fracture toughness and viscoelastic analysis. Poly Compos 2011;32(10): 1492–8. [33] Dalmas F, Cavaillé JY, Gauthier C, Chazeau L, Dendievel R. Viscoelastic behavior and electrical properties of flexible nanofiber filled polymer nanocomposites. Influence of processing conditions. Compos Sci Technol 2007;6:829–39. [34] Dalmas F, Chazeau L, Gauthier C, Cavaille JY, Dendievel R. Large deformation mechanical behavior of flexible nanofiber filled polymer nanocomposites. Polymer 2006;47:2802–12. [35] Mathew AP, Gong G, Bjorngrim N, Wixe D, Oksman K. Moisture absorption behavior and its impact on the mechanical properties of cellulose whiskers based polyvinyl acetate (PVAc) nanocomposites. Polym Eng Sci 2011;51(11):2136–42. [36] Lee S-Y, Mohan DJ, Kang I, Doh G, Lee Soo, Ok Han S. Nanocellulose reinforced PVA composite films: effects of acid treatment and filler loading. Fibers Polym 2009;10(1):77–82. [37] Hajji P, Cavaille JY, Favier V, Gauthier C, Vigier G. Tensile behavior of nanocomposites from latex and cellulose whiskers. Polym Compos 1996;17(4):612–9. [38] Sakurada I, Nukushina Y, Ito T. experimental determination of elastic modulus of crystalline regions in oriented polymers. J Polym Sci 1962;57(165):651–60. [39] Choi YJ, Simonsen J. Cellulose nanocrystal-filled carboxymethyl cellulose nanocomposites. J Nanosci Nanotechnol 2006;6:633–9. [40] Wang Y, Cao X, Zhang L. Effects of cellulose whiskers on properties of soy protein thermoplastics. Macromol Biosci 2006;6:524–31. [41] Lu Y, Wenig L, Zhang L. Morphology and properties of soy protein isolate thermoplastics reinforced with chitin whiskers. Biomacromolecules 2004;5:1046–51. [42] Dufresne A, Cavaillé JY. Clustering and percolation effects in microcrystalline starch-reinforced thermoplastic. J Polym Sci, Part B: Polym Phys 1998;36:2211–24. [43] Azeredo HMC, Mattoso LHC, Wood D, Williams TG, Bustillos RJA, McHugh TH. Nanocomposite edible films from mango puree reinforced with cellulose nanofibers. J Food Sci 2009;74(5):N31–5. [44] Angles MN, Dufresne A. Plasticized/tunicin whiskers nano composites. 1. Structural analysis. Macromolecules 2000;33: 8344–53. [45] Grunert M, Winter WT. Nanocomposites of cellulose acetate butyrate reinforced with cellulose nanocrystals. J Polym Environ 2002;10:27–30. [46] Bonini, C. Mise en evidence du role des interactions fibre/fibre et fibre/matrice dans des nanocomposites a‘ renfort cellulosique et matrice apolaire (atactique et isotactique). Ph. D. thesis, Joseph Fourier University, Grenoble, France; 2000. [47] Favier V, Canova GR, Shrivastava SC, Cavaillé JY. Mechanical percolation in cellulose whisker nanocomposites. Polym Eng Sci 1997;37:1732–9. [48] Dufresne A. Comparing the mechanical properties of high performances polymer nanocomposites from biological sources. J Nanosci Nanotechnol 2006;6:322–30. [49] Marchessault RH, Morehead FF, Walter NM. Liquid crystal systems from fibrillar polysaccharides. Nature 1959;184:632–3.
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A. Kaboorani et al. / European Polymer Journal 48 (2012) 1829–1837
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Nanocrystalline cellulose (NCC): A renewable nano-material for polyvinyl acetate (PVA) adhesive
a Département des sciences du bois et de la forêt, Faculté de foresterie, de géographie et de géomatique, Université Laval, 2425, rue de la Terrasse, Québec, QC, Canada G1V 0A6 b Nanotechnologies for Wood Products, FPInnovations, 319 rue Franquet, Québec, QC, Canada G1P 4R4 c CNR IVALSA, Consiglio Nazionale delle Ricerche – Istituto per la Valorizzazione del Legno e delle Specie Arboree I – via Biasi 75 – 38010 S. Michele all’Adige (TN), Italy d Center for Renewable Carbon, University of Tennessee, 2506 Jacob Drive, Knoxville, TN 37996-4570, USA
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 3 August 2012 Accepted 13 August 2012 Available online 19 August 2012 Keywords: Durable wood adhesives Polyvinyl acetate (PVA) Nanocrystalline cellulose (NCC) Wood joints Nanoindentation
a b s t r a c t In this study nanocrystalline cellulose (NCC) was used to improve the performance of polyvinyl acetate (PVA) as a wood adhesive. NCC was added to PVA at different loadings (1%, 2% and 3%) and the blends were used as binder for wood. Block shear tests were done to evaluate bonding strength of PVA at different conditions; dry and wet conditions, at the elevated temperature (100 °C). The mechanical properties of PVA film and its composites with NCC were measured by nanoindentation technique. Thermal stability and structure of nanocomposites were studied by thermogravimetric analysis and atomic force microscopy (AFM). The block shear tests demonstrate that NCC can improve bonding strength of PVA in all conditions. Hardness, modulus of elasticity (MOE) and creep of PVA film were also changed positively by the addition of NCC. Thermal stability of PVA was significantly improved as NCC was added to PVA. Structural studies revealed that variations in shear strength and other properties can be related to the quality of NCC dispersion in the PVA matrix. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently there has been a strong tendency to use so called ‘‘green materials’’ for the industry. The tendency has forced the industry to look for renewable materials with the least impact on the environment. One of the industries that can benefit from this trend and subsequently experience significant and constant growth is wood industry. Wood can provide the market with the prospects which cannot be offered by the other materials. Wood has a great potential to become the dominating material for the construction. As a material for the construction, wood has unique properties; such as high strength, flexibility, fire ⇑ Corresponding author. E-mail address: [email protected] (B. Riedl). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.08.008
resistance, durability, isolation and also not having carbon finger print. Introduction and development of engineered wood products (EWPs), which can be produced in different shapes and sizes, has opened new markets for wood industry and expand the current markets. Adhesives play an important role on the performance of EWPs. Production of EWPs with high performance needs durable adhesives. Commercial durable wood adhesives contain formaldehyde, causing concerns on the health during the production and the service of EWPs. Wood industry is under increasing pressure to eliminate formaldehyde from its products. Hence, wood industry is looking for alternatives. Polyvinyl acetate (PVA) is a good alternative to replace some wood adhesives containing formaldehyde. PVA is a linear and thermoplastic polymer. It is watersoluble, biodegradable with the excellent chemical
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Alireza Kaboorani a, Bernard Riedl a,⇑, Pierre Blanchet a,b, Marco Fellin c, Omid Hosseinaei d, Sequin Wang d
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resistance and has no toxic action on the human body. As a wood adhesive, utilization of PVA is very simple and its curing does not need high temperatures. The main drawback of PVA is its weak performance towards humid conditions and elevated temperatures. These drawbacks limit the usage of PVA in the applications where high performance is required in humid conditions and at elevated temperatures. So far two approaches have been used to increase the performance of PVA: (1) Copolymerizing vinyl acetate with more hydrophobic monomers or functional monomers [1–3], and (2) blending PVA with other adhesives or hardeners [4–13]. These strategies can increase some properties of PVA at the expense of reducing some other properties. Moreover, some additives are so acidic that they can damage wood subtracts, finally affecting the overall performance of wood joints. For instance, copolymers of vinyl acetate and butyl acrylate or ethylene can increase water resistance and toughness of the adhesive, but will reduce its tensile modulus and stiffness, especially at elevated temperatures [12]. The introduction of nanotechnology has opened new opportunities for the industries to develop a new generation of composites with high performance. Past research has showed that nano-clay and nano-aluminum oxide particles can be used to boost the performance of wood adhesives [14,15]. There has been always concerns on the health risks posed by nano-clay and nano oxide metal particles, however. Also the renewability of these kinds of nano-particles has been always under question. Such concerns gave a momentum to introduction of renewable and more eco-friendly nano-materials. Cellulose is the most abundant organic polymer in the biosphere and the main constituent of wood. Cellulose has a nano-structure which can be employed in nanotechnology. Cellulose fibrils consist of different hierarchical microstructures commonly known as nano-sized microfibrils. These nano-sized fibrils are again made-up of a combined crystalline and amorphous parts and this crystalline region is named in the literature nanocellulose or nanowhisker [16]. Nanocrystalline cellulose (NCC) is obtained by the acid hydrolysis of cellulose under conditions where the amorphous regions are selectively hydrolyzed. For woodbased NCCs, the remaining crystalline regions are 3– 10 nm in diameter and 100–300 nm long and retain the natural cellulose I crystalline structure [17,18].Among their interesting characteristics, NCCs are abundant and renewable. They exhibit also a low density as compared to mineral fillers (around 1.5 g cm3) and a high form factor of about 70 together with a high specific area of 150 m2 g1. They have been shown to lead to remarkable reinforcing properties in such different matrixes as styrene–acrylate latex [19], starch [20], polyhydroxybutyrate octanoate [21] or poly(ethylene oxide) [22]. In most cases, the reinforcing effects came from the percolating network of NCC together with good interfacial compatibility between the matrix and the fillers. The hypothesis of this study is that using NCCs as reinforcing nano-materials improves the performance of PVA as a wood adhesive. Such improvement can expand the applications of PVA, especially in the wood construction domain in which durable materials with high performance
are required. The main objective of this research is to develop durable eco-friendly wood adhesives with an application in construction field. The effects of adding NCCs on the mechanical properties, morphology and thermal stability of PVA will be discussed in this paper. 2. Materials and methods 2.1. Materials A commercial grade of polyvinyl acetate (PVA) was received in liquid form. Nanocrystalline cellulose (NCC), provided kindly by Forest Products Laboratory in Madison, WI. USA was used as reinforcing material, (in 5.5% suspension). Black spruce (Picea mariana Mill.), which were obtained from trees grown in province of Québec, Canada was used as wood substrates. 2.2. Preparation of nanocellulose/PVA composites A certain amount of the suspension of NCC was added to PVA, depending on the percentage of nanocellulose in PVA. The blends of PVA with nano-cellulose were mixed for 30 min. For TGA, nanoindentation and AFM, samples of nanocomposites were prepared by casting the NCC– PVA composites on Teflon sheets. Prior to further analyses, the sheets of nanocomposites were allowed to dry at room temperature for at least two weeks. 2.3. Fabrication and tests of wood joints Blends of PVA and NCC were used to bond wood joints. Black spruce was used as substrates. Prior to gluing, the moisture content of wood was fixed at 12% by conditioning wood to 20 °C and 60% relative humidity for four months. After applying glue on the surface of wood, the samples were pressed in an MTS hydraulic test machine with 50 kN capacity at 2.46 kg/cm2 pressure for three hours. Before testing, glued samples were conditioned to 20 °C and 60% relative humidity for two weeks. 20 samples were tested for each set of formulation. Shear strength of wood joints was measured in dry and wet states, and at an elevated temperature. An MTS hydraulic test machine with 50 kN capacity was used for load application and the data was acquired by a computer. Wood failure and maximum load were recorded for each test. The block shear tests were carried out according to ASTM D905-98. The sizes of samples for ‘‘wet state’’ tests were the same as those for dry state tests. For ‘‘wet state’’ tests, the samples were taken directly out of the water after being immersed in water for 24 h. Before the tests, excess water was wiped off from the samples. During the water immersion period, temperature of water was maintained at 23 ± 1 °C. Block shear tests at the elevated temperature were carried out according to ASTM 7247-07. Samples made of black spruce were heated in an oven, having a temperature controller with an integral and derivative (PID) control algorithm, until the temperature of middle of samples reached 100 °C. On average, it took 30 min to reach
A. Kaboorani et al. / European Polymer Journal 48 (2012) 1829–1837
2.4. Characterization of the nanocellulose/PVA composites
Er ¼
2.4.1. Nanoindentation Oliver and Pharr in 1992 introduced a technique, nanoindentation, to characterize the localized material properties [23]. Conventional measurements of mechanical properties only give a value for each test. In return, nanoindentation technique provides an opportunity to measure the mechanical properties of composites materials in desired areas. In this technique, the areas under test can be selected, giving different values depending on the place of tested areas. A typical nanoindentation profile for determination of localized mechanical properties in polymer composites is shown in Fig. 1. In the technique, a permanent plastic deformation is created in the surface of a material being tested. Hardness, elastic modulus (MOE) and creep of the material can be calculated by using the load-displacement data from the nanoindentation tests. For an indentation depth (h), the hardness (H) of a sample can be calculated from the following equation:
H¼
Pmax A
ð1Þ
where Pmax is the load measured at a maximum depth of penetration (h) in an indentation cycle, while A is to the projected area of contact between the indenter and sample at Pmax. In theory, the elastic modulus should be size independent [24], but analyzing polymers by NI causes a number of complications summarized under the term indentation size effect, which means that the elastic modulus tends to increase with decreasing penetration depth of the indenter pyramid. Also viscoelastic creep during unloading may affect the slope of the unloading curve, and thus the
Fig. 1. The loading profile of nanoindentation.
calculated elastic modulus. Comparing elastic moduli (E) of polymers from bulk measurements and indentation tests, results often show that Ebulk < Esurface [25]. From the initial slope of the unloading curve the unloading stiffness (S) is determined and the reduced elastic modulus Er is calculated according to Eq. (2).
1 pffiffiffiffi S p pffiffiffi 2 A
ð2Þ
Er is termed the reduced elastic modulus because it takes into account the compliance of the indenter tip according to Eq. (2).
1 m2i 1 1 m2m ¼ þ Er Em Ei material indenter
ð3Þ
The elastic modulus Em of the specimen was calculated according to Eq. (3) using Poisson’s ratios m determined in a previous study [26]. Ei and mi stand for the elastic modulus and the Poisson’s ratio of the indenter. Since the compliance of diamond is very small compared with that of the tested polymers, the term representing the indenter properties (subscript indenter) in Eq. (3) was disregarded.
C IT ¼
h2 h1 100 h1
ð4Þ
Indentation creep CIT (Eq. (4)) [27] was defined as the relative change of the indentation depth while the applied load remains constant (Fig. 1). To prepare the samples for the nanoindentation test, pure PVA and its composites with NCC were embedded in Spur epoxy resin. Spur epoxy resin is used as an embedding medium for electron microscopy of biological samples. The curing was performed in a vacuum oven at 70 °C for 7 h. An ultramicrotome (Leica, Vienna, Austria) was used to make a very smooth surface, by avoiding creating any default, on the surface of sample. A five-minute epoxy adhesive was used to glue the resin-embedded samples to an acrylic block. The acrylic block was then mounted onto an ultra-microtome. First a glass knife was used to level surface of the samples and then further on, creating a smooth surface was completed by a diamond knife. All nanoindentation experiments were performed on a Triboindenter (Hysitron, Minneapolis, MN) equipped with a three-sided pyramid diamond Berkovich tip. Before nanoindentation tests, samples were conditioned at 21 °C and 60% relative humidity for at least 24 h. Indentation was performed in a load-controlled mode using three segments. The loading time, holding time, unloading time and maximum indentation force (Pmax) are 5 s, 60 s, and 5 s and 150 lN for all testing samples, respectively. The separation distance between each nanoindentation point was 0.05 mm. 2.4.2. Thermal stability The thermal stability of pure NCC, pure PVA and their nanocomposites was studied using a thermogravimetric analyzer, Mettler Toledo TGA/SDTA 851e, on 13–15 mg of samples over a temperature range of 25–700 °C at a heating rate of 10 °C/min under a nitrogen flow rate of 50 mL/min. In order to avoid unwanted oxidation, thermal
MACROMOLECULAR NANOTECHNOLOGY
100 °C in the middle of samples. After reaching 100 °C in the middle of samples, the samples were kept at 100 °C for 15 min more, followed by the immediate block shear test. The shear strength of sample was measured by an MTS hydraulic test machine.
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stability measurements were conducted with the samples placed in high quality nitrogen (99.5% nitrogen and 0.5% oxygen content) atmosphere.
Ra ¼
n 1X jZi Zav ej n i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðZi Zav eÞ2 Rq ¼ RMS ¼ n
ð5Þ
ð6Þ
where Ra is the mean roughness, the arithmetic average of the absolute values of the surface height deviations, and Rq is the root mean square of the height and in both equations Zi is the current Z value, Zave is the average of the Z values within the given area and n is the number of points within the given area.
3.1. Bonding strength Fig. 2 shows shear stress at failure of wood joints bonded by pure PVA and its composites with NCC. Adding NCC to PVA did increase the stress but not in a significant way. Although not being significant in terms of the stress at failure, adding NCC to PVA significantly increased the wood failure, proving its positive effects on bonding
Shear stress [MPa]
80
6
60
4
40
2
20
Pure PVA
NCC 1%
NCC 2%
NCC 3%
0
Blend name Fig. 2. Shear strength and wood failure of joints bonded by PVA and its composites with NCC in dry state.
strength of PVA glue line. Wood failure value increased from 59% for pure PVA to 84% for adhesive with 1% NCC and finally to 97% for adhesive with 3% NCC. Such an increase shows that NCC made the glue line of PVA stronger than wood, causing failure in the wood rather than in the glue line. The loading level of NCC affected the performance of PVA. Wood failure percentage increased as more NCC was added to PVA. Sriupayo et al. [28], Kvien and Oksman [29], Zimmermann et al. [30], Favier et al. [31], Gong et al. [32] , Dalmas et al. [33,34], Mathew et al. [35] and Lee et al. [36] reported that nanocellulose positively affected the strength of the polymers. Favier et al. [31] stated that the intermolecular forces between nanocellulose and the PVA matrix may contribute to enhancement of strength of the PVA composite films. The intermolecular forces keep the inherent strength of the fibrils intact and results in enhancement of the mechanical strength of the films. Some reports have attributed the strong reinforcing effect of the NCC to the formation of a networked structure above percolation threshold resulting from hydrogen bonding [37,38].
8
100
7
Stress
90
Wood failure
80
6
70
5
60
4
50
3
40 30
2
20
1 0
Wood failure [%]
3. Results
8
0
2.5. Statistical analyses A one way analysis of variance model was used to study the effects of NCC loading on shear strength of wood joints. The GLM procedure of the SAS program was used and pair wise comparisons were then made using protected Fisher LSD (Least Significant Difference).
100
10
Shear stress [MPa]
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2.4.4. Atomic force microscopy (AFM) AFM observations were carried out using a NanoScope V, an atomic force microscope (Veeco Instruments Inc.), fitted with a Hybrid XYZ scanner. Atomic force microscopy measurements were done under ambient air conditions in tapping mode. The sensitivity of the tip deviation and the scanner resolution was 0.3 nm. The resolution was set to 250 lines by 256 pixels for all observations. Surface roughness was calculated in 10 lm 10 lm scan areas, using the classical mean surface roughness parameters Ra and Rq (RMS). The parameters were calculated by the Research Nanoscope 7.2 software:
120
Wood failure
Wood failure [%]
2.4.3. Transmission electron microscopy High-resolution transmission electron microscopy (TEM) was carried out with a JEOL 100 CX II microscope at 100 kV. The samples for TEM observations were prepared from a solution with 0.5% NCC. The samples were prepared by casting a drop of diluted aqueous dispersion onto a 200 mesh holey copper grid covered with a carbon film. To improve the contrast, the cellulose nanofibers were stained with a 1 wt.% solution of uranyl acetate for 30 s. The sample was then dipped into a droplet of distilled water for 2 s to remove the excess uranyl acetate. It was then allowed to dry at ambient temperature overnight.
Stress
10 Pure PVA
NCC 1%
NCC 2%
NCC 3%
0
Blend name Fig. 3. Shear strength and wood failure of joints bonded by PVA and its composites with NCC in wet state.
A. Kaboorani et al. / European Polymer Journal 48 (2012) 1829–1837
100
4 Wood failure
90
Studying the response of composites materials to a mechanical load in nano-scale helps to develop a better understanding of impact of nano-materials on mechanical properties of nanocomposites. Nanoindentation technique can provide us with an opportunity to understand the response of composites to a load in a nano-scale. Values of hardness and their variations in pure PVA and its composites with NCC are shown in Fig. 5. NCC addition to adhesive increased average values of hardness significantly. Adding only 1% NCC to PVA increased average value of hardness by 70%. At 2% NCC loading, the extent of improvement decreased and reached only 55%. An increase in NCC loading form 2% to 3% resulted in a huge increase in hardness (155%, more than double). Fluctuations in hardness within the composites were also affected by NCC. Variations in hardness were not very high in pure PVA. As NCC was inserted into PVA, the variations in hardness became high. The extent of variations in hardness was related to amount of the NCC in PVA. The higher the ratio of NCC to PVA, the greater the variation in hardness. Average values of MOE and their variations for each formulation measured by nanoindentation technique are shown in Fig. 6. Average values of MOE were affected significantly by addition of NCC. Adding 1% NCC to PVA increased average value of MOE by 48% but adding more NCC (2%) to PVA did not change MOE considerably. An increase in NCC loading from 2% to 3% resulted in an important increase in MOE. Film with 3% NCC showed the highest value of MOE. The variations of MOE in different parts of composites were influenced by adding NCC as well. The values of MOE in pure PVA were pretty constant. As NCC was added to the PVA matrix, the variations in MOE became more important. The highest variations in MOE were found in composites with 3% NCC. One of drawbacks of PVA is its high creep. Nanoindentation technique provides an opportunity to measure creep in different parts of film and analyze the effects of different fillers on performance of the polymer. Creep measurements conducted on the film of PVA and its composites demonstrated that NCC has a significant effect on average values of creep (Fig. 7). Adding NCC to PVA reduced the creep. As the percentage of NCC in PVA increased, the effect of NCC in reducing creep became more prominent. Adding
80
3
70 60 50
2
40 30
1
Wood failure [%]
Shear stress [MPa]
Stress
3.2. Nanoindentation
20 10
0
0
Pure PVA
0
0
NCC 1%
NCC 2%
0
NCC 3%
0
blend name Fig. 4. Shear strength and wood failure of joints bonded by PVA and its composites with NCC at the elevated temperature.
Fig. 5. Average values of harness for different PVA–NCC composites.
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The values of shear stress at failure of wood joints after spending 24 h in water are shown in Fig. 3. Nanocellulose affected significantly shear strength of wood joints after spending 24 h in water. Adding only 1% NCC to PVA increased the strength by 63%. Further increase in NCC (from 1% to 2%) increased shear strength of wood joints but on a much smaller scale. An increase in NCC loading from 2% to 3% did not change the strength much. The improvement on water resistance of PVA was so high that some failure occurred in glue line instead of wood. As more NCC was inserted in PVA, the percentage of wood failure increased, reaching its maximum value at 3% NCC level (21%). Sriupayo et al. [28], Choi et al. [39], Wang et al. [40], Lu et al. [41] and Dufresne et al. [42] reported a reduction in water uptake and diffusion coefficient of the polymers as nanocellulose added to the polymers. The high crystallinity of cellulose might be responsible for the decreased water uptake at equilibrium and the decreased diffusion coefficient of the materials. The presence of NCCs in the polymer can also create tortuosity in the nanocomposites films, leading to slower diffusion processes and, hence, to a lower permeability [43]. Fig. 4 shows the values of shear strength at 100 °C. NCC affected PVA performance at the high temperature significantly. Adding NCC to PVA doubled the shear strength of PVA. There was no significant difference between different loading levels of NCC. Adding only 1% NCC to PVA resulted in a 100% increase in shear strength and further addition did not change the strength much. Angles et al. [20] and Favier et al. [31] reported that incorporation of NCC in composites resulted in a significant increase in mechanical properties, especially at temperatures greater than the glass transition temperature. They attributed the reinforcing effect to the formation of a rigid NCCs network governed by the percolation threshold and strong hydrogen bonding between NCCs. It was demonstrated that the reinforcing effect of NCC lies in the formation of a rigid percolating filler network, due to hydrogen bond interactions between NCCs [44–46].
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Fig. 6. Average values of MOE for different PVA–NCC composites.
Fig. 7. Average values of creep PVA and its composites with NCC.
3% NCC led to 55% reduction in creep. Variations of creep were also affected by NCC. Pure PVA had a high variation in creep throughout the matrix. In the formulations with NCC, the variations in creep were less. The smallest variations in creep were observed in the formulation with 3% NCC. The results of measuring hardness, MOE and creep by nanoindentation technique demonstrate the great influence of NCC on the mechanical properties of PVA matrix. NCCs create an inhomogeneous matrix with different localized mechanical properties. Such a matrix shows improved mechanical properties, capable of bearing higher mechanical load as shown by bonding strength tests in this study. 3.3. Thermal stability Mass loss of PVA and its composites (measured in the range of temperature) was used as a tool to evaluate the effects of NCC on thermal stability of PVA. Thermal stability of PVA was affected by NCC (Fig. 8). The direction of changes in the thermal stability depended on the temperature. At temperatures less than 340 °C, PVA with NCC had better thermal stability than pure PVA. In temperatures between 340 and 420 °C, PVA and its composites had almost the same thermal stability. At temperatures higher than 420 °C, NCC negatively affected the thermal stability of PVA. No significant difference was detected between the nanocomposites having different amounts of NCC at temperatures less than 420 °C. Thermal stability of pure NCC
Fig. 8. Mass loss (a) and its rate (b) for PVA and its composites.
was similar to PVA composites in temperatures less than 235 °C. Between 235 and 340 °C, thermal stability of pure NCC is less than its composites with PVA. Thermal stability of pure NCC was better than its composites at temperatures higher than 340 °C. To have a detailed view over mass loss pattern occurred during thermal stability tests, the dM/dT curves obtained by derivation of mass loss/temperature data are given in Fig. 8b. The dM/dT curves show the speed of mass loss during thermal stability tests. Pure PVA has three peaks of weight loss at 258, 331 and 436 °C. As a result of adding NCC to PVA, one of the three peaks located at 258 °C disappeared. The second and biggest peak, located at 331 °C, remained almost untouched but the third peaks located at 436 °C occurred at little bit higher temperatures. It has
Table 1 Values of ash contents and DTGmax for pure PVA, pure NCC and their composites. Samples
DTGmax (°C)
Ash content (%)
Pure PVA Pure NCC PVA & 1% NCC PVA & 2% NCC PVA & 3% NCC
331.1 305.6 335.5 337.0 337.8
13.2 15.8 10.3 8.9 9.1
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been reported that the formation of a rigid network, resulting from strong interactions between NCCs and the polymer (governed by a percolation mechanism) [19,31], induced a thermal stabilization on the composite up to 227 °C, the temperature at which cellulose starts to
decompose. For rod-like particles with an aspect ratio of 67, the percolation threshold is close to 1 vol.% [47]. The formation of this cellulose network was supposed to result from strong interactions between NCCs, such as hydrogen bonds [48]. This mechanical percolation effect explains both the reinforcing effect and the thermal stabilization of the composite modulus for evaporated films. Values of DTGmax degradation temperature and ash contents for PVA and its composites are given in Table 1. DTGmax degradation temperature is a temperature in which the highest weight loss takes place (peak point in dM/dT curve). Pure PVA has higher DTGmax than pure NCC. Composites of NCC and PVA have higher DTGmax than pure PVA and pure NCC. It means that NCC can change the response of PVA towards temperatures. Ash content of PVA was also affected by adding NCC. Composites with NCC have lower ash contents than pure PVA. NCC proportion in PVA inversely affects the ash content of PVA. 3.4. Transmission electron microscopy Fig. 9 shows a TEM image of dilute suspension of NCCs. The NCCs have a rod-like structure. Acrylic latexes have been employed for a variety of applications including
Fig. 10. AFM amplitude mode analysis of PVA films with (a) 0%, (b) 1%, (c) 2 and (d) 3% NCC.
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Fig. 9. TEM image of NCCs.
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painting and adhesion. The suspension exhibited a colloidal behavior. This is due to the presence of the positive charges (NH3+) on the NCC surface, which resulted from the protonation of the amino groups of chitin in acidic conditions [49]. The suspension contains fragments consisting of both individual microcrystals and aggregated microcrystals. The rod-like NCCs were shown to have length values of 150–250 nm and width on the order of 4–10 nm, leading to an aspect ratio of 38–62.
NCC led to agglomeration of NCC and difficulty in dispersion of NCC in PVA at high loadings (2% and 3%). The agglomeration limits the extent of improvement that be achieved by NCC. Modifying the surface of NCC a little bit and imposing a slightly hydrophobic character on the surface can help to prevent agglomeration of NCCs and to have a better NCC dispersion in the matrix at a high loading. The modification of surface should be done in controlled conditions without making NCC too hydrophobic or degrading its reinforcing effects.
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3.5. Atomic force microscopy (AFM) To further explore the fundamental interactions between NCC and PVA, composite of PVA containing different loading of NCC (0%, 1%, 2% and 3%) were analyzed by atomic force microscopy (AFM). The AFM images for the composites are presented in Fig. 10. The images clearly demonstrate a re-organization of the surface of the composite films. Pure PVA film has a smooth surface as it has low Ra and Rq (Table 2). Adding 1% NCC to the matrix did not change Ra and Rq considerably, showing good dispersion of NCC at 1% loading. Adding 2% and 3% NCC to the matrix re-organized the surface as big jumps in Ra and Rq were detected. Such high Ra and Rq are evidence for poor quality of NCC dispersion in the matrix, and presence of aggregates. Seemingly high interaction between PVA and NCC (in the form of hydrogen bonding) prevented NCCs to be dispersed uniformly in the matrix. Comparing the results of morphological studies and mechanical and thermal properties displays that thermal stability and bonding strength (in wet conditions and at the elevated temperature) have more sensitivity to the quality of NCC dispersion in the PVA matrix. At 1% loading, NCC improved the thermal stability and bonding strength of PVA significantly. But adding more NCC (2% and 3%) did not increase the properties beyond the extent obtained by 1% addition. This trend is related to the quality of NCC dispersion. At 1% NCC loading, significant improvement on bonding strength and thermal stability was achieved as quality of NCC dispersion was good. At 2% and 3% NCC loadings, dispersion of NCC in PVA encountered difficulty, as shown by the AFM results, and a poor dispersion of NCC in the matrix was obtained leading to a leveling-off of the improvement. In the context of the others studies conducted on the reinforcement of PVA with inorganic nano-materials [13,14], the extent of improvement achievable by NCC is higher than that of inorganic nanomaterials, namely nano-clay and aluminum oxide. The improvement is especially much higher for thermal stability and bonding strength at wet conditions. In this study unmodified and hydrophilic NCC was used to reinforce PVA. Using such unmodified and hydrophilic Table 2 Values of surface roughness for pure PVA and its composites with NCC. Samples
Rq (nm)
Ra (nm)
Pure PVA PVA & 1% NCC PVA & 2% NCC PVA & 3% NCC
39.9 27.6 140 155
31.7 21.6 113 120
4. Conclusions NCC was found to be an effective nano-reinforcing material for PVA. Bonding strength of wood joints was improved significantly as NCC was inserted to PVA. The improvement was measurable in terms of wood failure percentage in dry condition and in terms of shear strength in wet condition and at the elevated temperature. NCC increased hardness and MOE of PVA film significantly. The creep experienced a big reduction as NCC was added to the PVA matrix (especially at high loadings of NCC). Thermal stability of PVA gained big enhancement by the addition. The three levels of NCC loading affected thermal stability in the same extent. Structural study of NCC–PVA composites revealed that NCC changed the structure of PVA. Fluctuations in properties of PVA as a wood adhesive and a polymer at different loadings of NCC could be explained by the quality of NCC dispersion in the matrix. The thermal stability and bonding strength of wood joints showed more sensibility to the quality of NCC dispersion in the matrix.
Acknowledgements The authors acknowledge financial support from Natural Sciences and Engineering Research Council of Canada. The authors would like to express their thanks to Forest Products Laboratory – USDA Forest Service for so kindly providing nanocrystalline cellulose (NCC) for this research. Thanks are also extended to FPInnovations for their assistance.
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